Formula Writing Examples in Chemistry

Start with the recap, study the fully worked examples, then use the practice problems to check your understanding of Formula Writing.

This page combines explanation, solved examples, and follow-up practice so you can move from recognition to confident problem-solving in Chemistry.

Concept Recap

The systematic process of combining element symbols and numerical subscripts to represent the exact composition of a chemical compound, ensuring that the total positive charge from the cations balances the total negative charge from the anions so the compound is electrically neutral.

Chemical formulas are the 'spelling' of chemistry — they tell you exactly which atoms and how many of each are in a compound.

Read the full concept explanation →

How to Use These Examples

Read the first worked example with the solution open so the structure is clear.
Try the practice problems before revealing each solution.
Use the related concepts and background knowledge badges if you feel stuck.

What to Focus On

Core idea: Formula Writing starts by identifying valence electrons, likely charges or sharing, and the structure that follows.

Common stuck point: Students often know a formula related to formula writing but skip the recognition step: Am I explaining a substance by electron behavior, bond type, molecular shape, polarity, or attractions between particles? That leads to a correct-looking substitution attached to the wrong chemical model.

Sense of Study hint: Ask: Am I explaining a substance by electron behavior, bond type, molecular shape, polarity, or attractions between particles?

Worked Examples

Example 1

easy

Write the chemical formula for aluminum chloride using the charges of the ions:

\text{Al}^{3+}

and

\text{Cl}^-

Answer

\text{AlCl}_3

First step

Aluminum has a charge of

+3

and chlorine has a charge of

-1

Full solution

2
To balance the charges, we need 3 chloride ions for every 1 aluminum ion: $3 \times (-1) = -3$ balances $+3$ .
3
The formula is $\text{AlCl}_3$ . The subscript 3 on Cl indicates three chloride ions per formula unit.

When writing ionic compound formulas, the total positive charge must equal the total negative charge so the compound is electrically neutral. The crisscross method (using charge magnitudes as subscripts) is a quick shortcut.

Example 2

medium

Write the chemical formula for calcium phosphate using the crisscross method. Calcium forms

\text{Ca}^{2+}

and phosphate is

\text{PO}_4^{3-}

Example 3

medium

Write the formula for copper(I) sulfide using the Stock system.

Example 4

hard

Combustion analysis of

0.500

g of a hydrocarbon yields

1.571

g CO

_2

and

0.643

g H

_2

O. Find its empirical formula.

Practice Problems

Try these problems on your own first, then open the solution to compare your method.

Example 1

medium

Write the correct chemical formulas for: (a) iron(III) sulfate, (b) ammonium carbonate.

Example 2

hard

A student writes the formula for magnesium nitride as

\text{MgN}

. Explain why this is incorrect and determine the correct formula. (

\text{Mg}^{2+}

\text{N}^{3-}

)

Example 3

easy

Write the formula for sodium chloride from

Na^+

and

Cl^-

Example 4

easy

Write the formula for

Mg^{2+}

and

Cl^-

Example 5

easy

Write the formula for

Al^{3+}

and

O^{2-}

Example 6

easy

Write the formula for calcium hydroxide (

Ca^{2+}

and

OH^-

Example 7

easy

Write the formula for potassium oxide (

K^+

and

O^{2-}

Example 8

easy

Write the formula for ammonium chloride (

NH_4^+

and

Cl^-

Example 9

easy

Write the formula for magnesium oxide (

Mg^{2+}

and

O^{2-}

Example 10

easy

Write the formula for sodium sulfate (

Na^+

and

SO_4^{2-}

Example 11

medium

Write the formula for aluminum sulfate (

Al^{3+}

and

SO_4^{2-}

Example 12

medium

Write the formula for iron(III) oxide.

Example 13

medium

Write the formula for calcium phosphate (

Ca^{2+}

and

PO_4^{3-}

Example 14

medium

Write the formula for ammonium sulfate (

NH_4^+

and

SO_4^{2-}

Example 15

medium

Write the formula for copper(II) nitrate.

Example 16

medium

Distinguish: in

3H_2O

, what does the 3 represent versus the 2?

Example 17

medium

Write the formula for iron(II) phosphate.

Example 18

medium

Why must

Al_2O_3

(not

Al_2O_2

) form from

Al^{3+}

and

O^{2-}

Example 19

medium

Write the formula for sodium phosphate (

Na^+

and

PO_4^{3-}

Example 20

challenge

A metal M forms

M_2(SO_4)_3

. What is the charge on M?

Example 21

challenge

Write the formula for chromium(III) sulfate and total the atoms.

Example 22

challenge

Element X is in group 2 and Y is in group 16. Predict the compound formula.

Example 23

easy

Write the chemical formula for lithium fluoride (

\text{Li}^+

and

\text{F}^-

Example 24

easy

Write the chemical formula for barium chloride (

\text{Ba}^{2+}

and

\text{Cl}^-

Example 25

easy

Write the formula for zinc oxide (

\text{Zn}^{2+}

and

\text{O}^{2-}

Example 26

easy

Write the formula for potassium bromide.

Example 27

medium

Write the formula for sodium phosphide (

\text{Na}^+

and

\text{P}^{3-}

Example 28

medium

Write the formula for aluminum nitrate (

\text{Al}^{3+}

and

\text{NO}_3^-

Example 29

medium

Write the formula for magnesium phosphate.

Example 30

medium

Write the formula for tin(IV) oxide.

Example 31

medium

Write the formula for potassium permanganate (

\text{K}^+

and

\text{MnO}_4^-

Example 32

medium

Write the formula for ammonium phosphate.

Example 33

medium

Write the formula for calcium carbonate.

Example 34

medium

Write the formula for lead(II) nitrate.

Example 35

medium

Write the formula for iron(III) hydroxide.

Example 36

hard

A compound has the empirical formula CH

_2

O and a molar mass of

180\,\text{g/mol}

. Write its molecular formula.

Example 37

hard

A sample contains

40.0\%

S and

60.0\%

O by mass. Find its empirical formula.

Example 38

hard

Write the formula for chromium(III) phosphate.

Example 39

hard

Write the formula for ammonium dichromate (

\text{NH}_4^+

and

\text{Cr}_2\text{O}_7^{2-}

Example 40

hard

Write the formula for the binary covalent compound dinitrogen pentoxide.

Example 41

challenge

Hydrated copper(II) sulfate has formula

\text{CuSO}_4\cdot x\text{H}_2\text{O}

and is

36.1\%

water by mass (

M_{CuSO_4} = 159.6

M_{H_2O} = 18.0

). Find

x

Example 42

challenge

A binary ionic compound of element X (group 13) and element Y (group 16) is electrically neutral. Predict X's charge, Y's charge, and the formula.

← Back to Formula Writing Practice Mode

Related Concepts

Ion Chemical Bond Nomenclature Chemical Equation

Background Knowledge

These ideas may be useful before you work through the harder examples.

ionchemical bond